Seems ok, but I wonder about a sensor that has a 5.62 crop factor with a pixel count of 16 mp, compared with the Sony's 2.7 crop factor, 20 mp sensor. If you crop the image from the Sony to the same size as the Nikon, your field of view is similar to an 1800mm lens more or less, but you'd lose some pixels in the process. All in all,the small sensor size makes the Nikon less desirable for landscape, but I suppose OK for closer work in good light. Also at full extension the Sony RX10M3 or M4 are F4, the Nikon is F8. A field of view similar to a 3000mm lens on an FX camera will be pretty challenging to hold still. I suspect a pretty substantial tripod, not a Dolica, meFoto, Vanguard, Ravelli - the tripod and head will likely cost twice what the camera costs.
The P1000's AF is a slower, contrast detect system, making it unsuitable for fast moving subjects. The Sony has 315 very fast phase detect focus points. It is the fastest focusing camera I have ever used, except for the other Sonys.
However, I do like the flexible rear screen, especially if you are going to use this for video. Sony blew it on that count. Also, the Sony cannot zoom and focus at the same time.
The Nikon is rated for 250 shots per charge, the Sony is 400.
The Sony will shoot a burst at 24 fps, with a buffer big enough to shoot 217 jpegs or 112 raw files, the Nikon tops out at 7, with a buffer of only 7 images.
The Sony is 5/8 lb lighter - for those who are concerned with weight.
Sony offers shutter speeds from 30 secs to 1/32000 sec.
I wouldn't trade my Sony RX10M4 for one of these - focal length isn't everything a long lens without a fast capable camera behind it seems more like a marketing ploy than anything else. The Panasonic FZ2000 / FZ2500 is a serious alternative to the Sony RX10M4, the Nikon is a different class of camera with it's smaller sensor, slower handling, limited battery life, etc. It does look like a few things have been improved over the P900, but I didn't like that camera either. FWIW I do shoot Nikon FX - D800/D810.
My opinion is entirely based on the printed specifications on Nikon's website - obviously I have not had one in my hands.